1. Field of the Invention
The present invention relates to a technology for reproducing images, and it particularly relates to method and apparatus for reproducing coded image data, and to a television receiver that utilizes said method and apparatus.
2. Description of the Related Art
The digitalization of TV broadcasting is advancing by rapid strides. The digital broadcasting has already begun in BS (Broadcast Satellite) broadcasts and CS (Communication Satellite) broadcasts, and the plan is set for the digitalization of ground wave broadcasts as well. In digital TV broadcasting, the use of MPEG-2 (Moving Picture Expert Group 2), which is an international standard for data compression and expansion, makes it possible not only to transmit and store information at high efficiency but also to transmit multiple channels by a single repeater. It is also expected to provide greater convenience to the users.
On the other hand, the widespread use of portable terminals in recent years is expected to create greater needs for coding systems with high data compression ratio. Accordingly, investigations are being conducted on the use of MPEG-4 coding methods that can transmit images compressed at low bit rates. In digital TV broadcasting from now on, it seems that MPEG-4 will be used along with MPEG-2 for the distribution of image information.
In the digital TV broadcasting as mentioned above, it is necessary to extract coded image data from received signals and decode them. In the image coding by MPEG-2, inter-frame coding is done together with intra-frame coding, so that decoding according to the standard can not be carried out immediately after the reception of coded image data, when it is impossible to refer to the reference frames for inter-frame coding.
To resolve this problem, the inventor of the present invention proposed, in Japanese Patent Application Laid-Open No. 2002-112138, a digital broadcast receiving apparatus capable of receiving in the background and storing in advance one of a plurality of channels in preparation for the channel switching by the user, but has further come to realize that additional demands described as follows exist in connection with the subject matter. That is, in the image coding by MPEG-4, the data compression ratio is raised higher by holding the number of reference frames lower than MPEG-2 in order to increase the proportion of inter-frame coding, with the result that the delay time for coding is even longer. When the MPEG-4 system is employed for digital TV broadcasting, reference frames cannot be received for about four seconds, at the worst, after the start of reception, during which decoding of images cannot be started. Hence, images cannot be displayed immediately at power-on or channel switching of the receiving apparatus such as a television receiver, which will surely irritate the user. This is a great demerit for a product to be marketed.